The present invention relates to a projection aligner for projecting a mask pattern formed on a mask onto an object to be exposed to transfer the mask pattern to the object.
Projection aligners have been used to form wiring patterns of PCBs (Printed Circuit Boards), for example. A typical projection aligner is provided with a high-power light source such as an ultra-high-pressure mercury-vapor lamp which is provided to emit a light beam toward a mask, and a projecting optical system to project the light beam passed through the mask so that a substrate of the PCB is exposed by the beam passed through the mask and an image of the mask pattern is transferred.
The PCB usually includes through holes for electrically connecting wiring patterns formed on different surfaces thereof. The projection aligner have to adjust the positions of the mask and the substrate such that the mask pattern is transferred to the substrate at a correct location relative to the through holes previously formed to the substrate to ensure electrical connection between the wiring patterns.
The substrate of the PCB, however, expands/contracts due to, for example, temperature variation and/or surface preparation such as mechanical polishing. Such expansion/contraction of the substrate causes displacement of the through holes and, in turn, positional mismatch between the through holes and the mask pattern transferred to the substrate.